1. Field of the Invention
This invention relates generally to nuclear fuel handling apparatus and, more particularly, to a remotely controlled hand tool which is adapted for use during refuelling or inspections in order to mount and/or remove fastener bolts used to secure a channel box to a nuclear fuel assembly. This invention is particularly useful for nuclear fuel assemblies for a boiling water type nuclear reactor.
2. Background
Conventional designs of fuel assemblies for use in a boiling water type nuclear reactor include a multiplicity of fuel rods held in an organized array by spacers spaced along the fuel assembly length. As illustrated in FIG. 1 of the accompanying drawings, such a fuel assembly 10 includes a tubular channel box 12 which is generally square in cross section having a fuel assembly proper 14 mounted therein in a telescopic relationship to each other. The channel box 12 serves to promote contact of the fuel rods with primary coolant flow, facilitate guidance of the fuel rods in the reactor and protect the fuel assembly against accidental damage during transfer to and from a spent fuel storage pool. The channel box 12 is fastened to the fuel assembly by means of a single fastener bolt 16.
A plurality, e.g., about 760 such fuel assemblies are used in a light water reactor of a million KW class, and usually about one fourth of the 760 assemblies, i.e., about 200 assemblies are replaced once a year during a refuelling operation.
During refuelling or periodically performed inspections, such fuel assemblies 10 are transferred from the reactor to a spent fuel storage pool where the fastener bolt 16 is removed using a special tool, commonly called a "bolt wrench", for subsequent separation of the channel box 12 from the associated fuel assembly. The channel box and the fuel assembly proper are then processed separately. This handling of the spent fuel assemblies is usually carried out underwater within the spent fuel storage pool for purposes of reducing operator's exposure to radioactivities and of cooling the fuel assemblies.
FIGS. 2(a) and 2(b) are a partially sectioned front elevation and a perspective view, respectively, of a typical fastener bolt 16 comprising a bolt proper 18, an angle guard 20 and a bifurcated spring washer 22. The bolt proper 18 has a spline head 18a, a circumferential groove 18b, a neck 18c and a threaded portion 18d. The angle guard 20 includes a flat top and an angle portion integrally formed therewith. The angle guard 20 and the bifurcated spring washer 22 are loosely fitted around the bolt neck 18c so that they may not fall off from the fastener bolt. As shown in FIG. 1, the fastener bolt 16 is placed on a clip 24 provided at a top corner of the channel box 12, with the angle portion of the guard 20 and the bifurcated washer 22 disposed on the outer surfaces of the channel box, and also with the threaded portion 18d extending through an opening in the clip 24. To secure the channel box 12 to the fuel assembly, the fastener bolt 16 is rotated in a direction to move the thread portion 18d into a threaded bore formed in the top of the fuel assembly.
FIG. 3 illustrates a typical tool in the form of a bolt wrench 30 of the conventional type used to remove the fastener bolt 16 from the spent fuel assembly 10. This bolt wrench 30 is about 4 meters long and includes a control rod 32 having a knob 34 provided at its upper end and a cylindrical socket wrench 36 mounted at its lower open end; a leaf spring 38 mounted around the socket wrench 36 and having an inwardly directed annular projection 40 near its lower end; a cylindrical cover 42 for the control rod 32, the cover supporting at its lower end a mouth piece 44 having a tapered bore 46; and a guide member 48. This bolt wrench 30 is usually employed in the manner to be described below:
(1) From a platform above the spent fuel storage pool, the bolt wrench 30 is lowered into the water by an operator on the platform until the lower end of the bolt wrench 30 rests on one of the underwater fuel assemblies 10. The bolt wrench 30 is further lowered with the guide member 48 in sliding engagement with the outer surfaces of the channel box 12, as shown in FIG. 3, until the socket wrench 36 on the lower end of the control rod 32 is received on the spline head 18a of the fastener bolt 16.
(2) Then, the knob 34 is rotated in a direction to loosen the bolt 18 from the threaded bore and simultaneously to move the socket wrench 36 and the control rod 32 upwardly relative to the cover 42 until the annular projection 40 of the leaf spring 38 moves radially inwardly into engagement with the circumferential groove 18b of the fastener bolt 18 because of the leaf spring 38 being in sliding engagement with the inner wall of the tapered bore 46. In this condition, it will be appreciated that the fastener bolt 18 is firmly held by the bolt wrench 30 so that it is ready for transfer to a predetermined location.
While the bolt wrench of this conventional type has proved generally satisfactory, it has several drawbacks as explained below:
Because of its structural limitations, the bolt wrench cannot be made longer than 5 meters, for example, so that in some installations where fuel assemblies are stored at a considerable distance from the operating platform, it is impossible to remove fastener bolts using this bolt wrench. To solve this problem, it has been a common practice to transfer fuel assemblies temporarily stored on a fuel rack which is positioned deep enough to have the upper ends of the fuel assemblies thereon about 7 meters below the water level to a preparation machine which is positioned lesser depth than the fuel rack, in order to perform the fastener bolt removing operation at the preparation machine. However, this necessitates frequent transfer of fuel assemblies within the spent fuel storage pool and thus increases the likelihood of accidental damage to the fuel assemblies during transfer. Further, more manpower will be needed in performing this operation and the level of operator's exposure to radioactivities will be increased.
Certainly it has been proposed to lengthen the control rod 32 of this bolt wrench 30 to such an extent that the socket wrench 36 on the lower end thereof can reach the fastener bolt to be removed. However, it should be noted that there is a need to reduce the weight of the control rod 32 and the cover 42 therefor to provide an overall light-weight yet sturdy structure which facilitates handling of the bolt wrench. Accordingly, further lengthening of the control rod 32 will result in unwanted bending of the rod during transportation or bolt removing, thus preventing its practical use.
There are additional problems associated with the above-described bolt wrench 30. For example, the annular projection 40 of the leaf spring 38 slidably engages the fastener bolt 16 for holding it firmly for subsequent transfer. Since the leaf spring 38 is formed of a relatively thin material, the annular projection 40 becomes excessively worn in a short period of time, necessitating frequent replacement of the leaf spring. Difficulties are also experienced by the operator in determining if the bolt wrench 30 is properly gripping the fastener bolt 16. If the fastener bolt is not properly gripped by the wrench, there is the likelihood that the bolt 16 will fall off during transfer.
FIG. 4 is a top plan view showing the manner in which the fuel assembly 10 is placed on the fuel rack in the spent fuel storage pool. In certain installations, the walls of the fuel rack have a height less than the channel box 12 of the fuel assembly 10, so that when placed on the fuel rack, the channel box projects above the walls 50. Accordingly, the operator does not usually have difficulties in bringing the bolt wrench of the prior art to an operative position on the fuel assembly. However, in most installations, the walls of the fuel racks are taller than the channel boxes 12 to be stored. In addition, the fuel assemblies are placed on such fuel racks with their channel boxes 12 in close contact with the inner walls of the fuel rack, so that the head 18a of the fastener bolt 16 threaded into the clip 24 is not adequately spaced from the two adjacent rack walls, as seen in FIG. 4. As a consequence of this inadequate spacing, the operator usually suffers from great difficulties in lowering the bolt wrench 30 into turning engagement with the head 18a of the fastener bolt 16, since the bolt gripping structure surrounding the socket wrench tends to come into abutting engagement with the upper edges of the fuel rack walls, preventing further lowering of the bolt wrench 30 into its operative position to grip the head 18a of the fastener bolt 16.
It is the primary object of the present invention to provide a new and improved apparatus for removing a fastener bolt used to secure a channel box to the fuel assembly with a view to overcoming the above-stated disadvantages of the prior art apparatus.
Another object is to provide an improved bolt wrench which enables an efficient and reliable bolt removing operation irrespective of the relative height of the channel box to the walls of the fuel rack on which the fuel assembly is placed.
A further object of the invention is to provide an improved bolt wrench which is capable of efficient bolt removal without the need to transfer fastener bolts from the fuel rack to a preparation machine.
A still further object is to provide an improved bolt wrench which can be divided into three shorter sections which are easy to handle for storage and transportation and which can easily be modified into a longer wrench by adding at least one intermediate section to the overall structure.